1. Field of the Invention
The present invention relates to an apparatus for measuring pressure inside a predetermined vessel based on the principle that the transmitting efficiency of ultrasonic waves is changed by acoustic impedance variation according to an internal pressure. The present invention provides a pressure measuring apparatus which can improve resolutions, increase a dynamic range, and measure pressure in a low or high vacuum state and even under atmospheric pressure or higher, including a high pressure, by disposing ultrasonic transducers and/or a reflection plate inside a vessel or inducing resonance.
2. Background of the Related Art
Technologies for measuring pressure inside a predetermined vessel are generally used to check the degree of vacuum inside the vessel in the semiconductor or display manufacturing process. Among various methods to measure pressure, a capacitance diaphragm gauge (CDG) is mostly used. The capacitance diaphragm gauge is based on the principle that the displacement of the diaphragm of the capacitance diaphragm gauge is changed by a variation in the internal pressure of a vessel, and hence capacitance is changed. In general, the capacitance diaphragm gauge is available only in a low vacuum region.
To measure pressure in a high vacuum region an ionization gauge is usually employed. The ionization gauge is based on the principle that, when pressure is changed, a probability that electrons may collide against gas molecules increases, and the number of generated positive ions changes when the electrons collide against the gas molecules. The ionization gauge can measure pressure in the range of high vacuum regions of 10−1 Pa to 10−10 Pa, but has a problem that linearity cannot be guaranteed below 10−6 Pa.
As for high pressure gauges, pressure in a limited small pressure chamber of various ultra-high pressure generators must be measured directly. An ultra-high pressure is generated by compressing a sample by application of a force to a sample compression device, such as a piston-cylinder type, using a hydraulic pressure device. Accordingly, an average pressure of the sample can be found by dividing the magnitude of applied force by a cross section of the sample chamber cross section. However, if this method is used, pressure distributions inside a gasket for sealing a sample are not constant, loss of a force due to friction is increased, and therefore pressure values are only approximately estimated.
As described above, there is a problem in that a pressure measuring apparatus in which an approximate degree of vacuum of a vessel is taken into consideration has to be selected and installed every time. Further, measurement methods are changed, resulting in inconvenience in use.